Many industrial waste streams possess characteristics which restrict or preclude their discharge to municipal treatment facilities. Contaminants in wastewater may be organic or inorganic in nature and often are found in combination with one another. Some of the parameters which are regulated are the solution's chemical oxygen demand (COD), total organic carbon (TOC) and dissolved organic carbon (DOC). There are also many specific compounds and classes of compounds which are regulated. Examples of these are toxic ions such as cyanide and classes of toxic organic materials such as phenols. Electrochemical oxidation is a convenient technique for reducing the amount of undesirable organic compounds and other oxidizable species in a given solution to a level which is acceptable for discharge to a treatment facility.
Environmental regulations are becoming stricter around the world. Some effluents which were once sewerable must now be collected and hauled away for disposal, putting additional economic stress on manufacturers. It is therefore desirable to have a simple and efficient way of treating effluents in a way which will enable them to be discharged directly to the sewer.
The electrolytic treatment of wastewaters has been the subject of many patents, journal articles and technical presentations over the last few years. See, for example, U.S. Pat. Nos. 4,014,766; 4,399,020; 4,308,122; 4,839,007; and 5,160,417 and Gattrell, M. and Kirk, D. W., "The Electrochemical Oxidation of Aqueous Phenol at a Glassy Carbon Electrode" Can. J. of Chem. Eng., vol. 68 (Dec. 1990) pp. 997-1001. The advantages of electrolytic oxidation of wastes over chemical or thermal processes are the ease of operation, simplicity of design and relatively small equipment space requirements. Electrolysis is also considered to be relatively safe to operate when compared to oxidative treatment techniques which necessitate handling powerful chemical oxidants.
However, there are a number of problems and drawbacks associated with many known methods of electrolytic oxidation of solutes in wastewaters. Such problems and drawbacks appear to result in part from the particular materials which constitute the anodes employed in such electrolytic methods.
Most anode materials gradually corrode during use in electrolytic oxidation, especially in harsh chemical environments. Corrosion of typical anodes such as platinum, ruthenium dioxide, lead dioxide and tin dioxide leads to discharge of toxic materials into the environment. Secondly, non-renewable metal resources are consumed. Platinum anodes are the most acceptable of the traditional electrodes. In practice, the rate of loss of platinum from the electrode is high enough that a metal recovery system such as ion exchange would be required to remove the platinum from solution both for regulatory and economic reasons. The higher overall cost of such a system combined with the added level of complexity would severely limit the usefulness of the electrolytic oxidation treatment technique.
Tin dioxide on a conductive substrate shows promise as an anode; however, passivation of this electrode occurring at the tin/substrate interface has been cited as a mode of failure. See, for example, Koetz et al., Journal of Applied Electrochemistry, 21 (1991) pp. 14-20.
Also, many known anode materials (e.g., platinum) tend to become fouled during electrolytic oxidation of various solutes (e.g., phenols) by the formation of an adsorbed layer of residue on the working surface of the anode, which lowers the effectiveness and shortens the useful life of the anode, resulting in lengthier treatment time, more down time, and higher overall expense for electrolytic methods.
Furthermore, most known anode materials exhibit lower-than-desirable energy efficiency when employed in electrolytic oxidation, requiring relatively lengthy time and relatively high amounts of energy expenditure to achieve desired results at electrical current densities typically employed.
Also, when attempts are made to increase the rate of electrolytic oxidation by raising the current density at the working surface of many typical anodes, there is often a corresponding decrease in energy efficiency of the anodes, which at least partially offsets the effort to improve oxidation rate by raising current density and increases the amount of energy expenditure required.
Another drawback of prior art electrolytic oxidative methods, in regard to attempts to treat a wide range of different solutes that may be found in industrial wastewaters, is that anodes commonly employed in such attempts, e.g., platinum anodes, have been found by the present inventors to be so energy inefficient in treating some solutes, that they can be considered virtually ineffective at oxidizing such solutes, e.g., chelating ligands such as phosphonates or hydroxycarboxylic acids that are often included in various photographic solutions.
Also, while some electrolytic methods employing typical anodes have some effect on certain types of solutes, it is not the desired effect. For example, attempts to employ typical platinum anodes to electrolytically treat solutions containing mixtures of dissolved phenols and halide ions have been found by the present inventors not to result in the complete oxidation of the phenols. Rather, undesirable side reactions occur that cause the formation of halogenated hydrocarbons that precipitate out of the solution and then must be dealt with by some other means in addition to the attempted electrolytic oxidation.
Therefore, there is a continuing need for a method of electrolytic oxidation of solutes in liquid solutions that will avoid or minimize the problems and drawbacks described above. That is, a method is needed wherein: the anode employed does not itself discharge toxic or non-renewable metal resource materials into the solutions; the anode does not tend to become fouled and lower its effectiveness and useful life; the anode enables the method to be carried out with relatively high energy efficiency, both at electrical current densities typically employed heretofore and at current densities significantly higher than those typically employed; and the anode enables the method to be effectively applied to a wide range of different solutes in an energy-efficient manner and without causing extensive undesirable side reactions that would prevent the complete oxidation of the solutes.